1. Technical Field
The present embodiments relate generally to the field of railroad couplers, and more specifically, to the manufacture of an improved knuckle through the use of an improved sand core.
2. Related Art
Railcar couplers are disposed at each end of a railway car to enable joining one end of such railway car to an adjacently disposed end of another railway car. The engageable portion of each of these couplers is known in the railway art as a knuckle.
Typically, a knuckle is manufactured by a mold and several cores that are disposed within the mold. The mold shapes the outside of a casting. The cores are disposed to shape the inside or outside of a casting. Without the inside cores, the casting would be made of solid metal. The outside cores help shape the exterior of the casting. The inside cores commonly are referred to as a finger core in the front portion of the knuckle, a pivot pin core in the center of the knuckle, and a kidney core at the rear of a knuckle, and form the cavities in the knuckle upon casting.
During the casting process itself, the interrelationship of the mold and three cores (sometimes manufactured as one or two piece cores) disposed within the mold are critical to producing a satisfactory railway coupler knuckle. Many knuckles fail from internal and/or external inconsistencies in the metal throughout the thickness of the knuckle. If one or more cores move during the casting process, then some knuckle walls may end up thinner than others, resulting in offset loading and, in turn, resulting in an increased failure risk during use of the knuckle.
The external features of a coupler knuckle should meet railroad industry standards both because of initial acceptance of the knuckle and for its successful performance in service. One external feature that must be formed properly for successful knuckle performance in service is the pulling face contour. The pulling faces of mating couplers contact each other when freight cars are coupled together and transmit the forces pulling the train. The pulling forces within a train can be substantial. For this reason, railroad industry standards exist that specify the shape of the pulling face contour. Inconsistent or out of tolerance pulling face contours can result in poor coupling/uncoupling performance of the coupler or in detrimental load paths for the pulling load. One patent that discusses the importance of the proper performance of the pulling face is U.S. Pat. No. 7,337,826 entitled “Railway Car Coupler Knuckle Having Improved Bearing Surface.” The '826 patent describes techniques for casting a knuckle coupler with an enhanced bearing surface. The '826 patent, however, is silent regarding as to addressing the imperfections that can form on the knuckle during casting.
Coupler knuckles are generally manufactured from cast steel or alloys. Typically, silica sand or silica sand derivatives known in the art are used to create the mold walls and the cores. Such sands, however, have several potential drawbacks, which can adversely affect the knuckle's surface finish or its ability to maintain required dimensional control, which in turn can lead to the premature failure of the knuckle and increase maintenance costs as a result of premature failure.
By way of example, when a molten metal is introduced into a mold during casting, it is prone to shrinking as it cools and solidifies. This is known as “shrinkage” or “micro-shrinkage” and occurs because most metals are less dense as a liquid than as a solid. Shrinkage may occur on the outside of the casting, the inside of the casting, or both. Shrinkage may lead to the knuckle to form shrinkage defects and even a void in certain portions of the knuckle. This could cause the coupler to prematurely wear or result in premature fatigue and/or failure.
One technique used to overcome micro-shrinkage is the inclusion of risers in the mold to feed the volumes of the casting that are prone to shrinkage with additional casting material as it cools. However, once the knuckle is cast, the risers must be removed, typically by surface grinding. This may cause damage to the knuckle's surface and cause the knuckle to prematurely fatigue and/or fail. Moreover, risers and/or large ingates, i.e., material that connects the risers to the casting, are limited in their ability to provide for a uniform thickness throughout the casting, maintain precise part profile, and lose their effectiveness in areas further away from the riser.
Another technique used to address micro-shrinkage issues is the addition of metal chills. These may be external chills, which may be placed along the mold walls at predetermined locations, or may be internal chills. Internal chills can be pieces of metal that are strategically placed inside the mold cavity and ultimately become part of the casting. Chills absorb and remove the heat from the poured metal in the location of the chill in order to promote solidification and limit the amount of shrinkage in the vicinity of the small area in which they are located. External chills, however, may leave scars or other defects on a casting's surface that requires the casting to undergo extra finishing operations such as grinding, which may adversely affect the knuckle's surface finish. External chills add additional cost, and due to their manual application can result in inconsistent quality. Sometimes personnel inadvertently neglect the installations of chills or place them in the incorrect location. Internal chills add cost because they must be made of the same material, or at least compatible, with the casting. Moreover, chills may not fuse properly with the casting, thus causing premature failure or again requiring the casting to undergo a further finishing and/or repair process. Moreover, chills must be clean and free of rust or other impurities so as not to inhibit the solidification process.
Another drawback associated with silica sand and its derivatives is their higher rate of thermal expansion during the casting process. This may cause the mold to develop buckles and ultimately crack, such that the molten metal will enter the crack and create a fin projecting from the casting surface (also known as a “vein”). It is preferred that these veins are removed, again typically through a grinding process, which again may result in fatigue failures, and correspondingly increases the finishing cost.